1. Field of the Invention
The invention pertains to the field of closed loop chain drive tensioners for application with internal combustion engines. More particularly, the invention pertains to a dual tensioner that pivots about a single mounting point.
2. Description of Related Art
A tensioning device is used to control a closed loop power transmission chain as the chain travels between a plurality of sprockets that are connected to the operating shafts of an internal combustion engine. In this system, the chain transmits power from a driving shaft to a driven shaft, such as a camshaft, so that, at any point in time, part of the chain may be slack while part is tight. It is important to impart and maintain a certain degree of tension on the chain to prevent noise, slippage, or the unmeshing of teeth as in the case of a toothed chain drive system. Prevention of such slippage is particularly important in the case of a chain driven camshaft in an internal combustion engine because the jumping of teeth will throw off the camshaft timing, possibly causing damage to the engine or rendering it totally inoperative.
However, in the harsh environment of the internal combustion engine, numerous factors cause fluctuations in the tension of any given portion of the chain. For instance, extreme temperature fluctuations and differences in the coefficients of thermal expansion between the various parts of the engine can cause the chain tension to vary between excessively high and very low levels. During prolonged use, wear to the components of the power transmission system can cause a steady decrease in chain tension. In addition, camshaft and crankshaft induced torsional vibrations cause considerable variations in chain tensions. For example, the reverse rotation of an engine, occurring during stopping of the engine or in failed attempts at starting the engine, can also cause significant fluctuations in chain tension. For these reasons, a mechanism is desired to remove excessive tensioning forces on the tight side of the chain while, at the same time, ensuring that adequate tension is applied to the slack side of the chain.
Hydraulic tensioners are a common method of maintaining proper chain tension. In general, these mechanisms employ a lever arm that pushes against the chain on the slack side of the power transmission system. Hydraulic pressure urges a piston against the lever arm, which in turn, forcibly engages the chain to tighten it during slack conditions.
Blade tensioners are commonly used to control a chain or belt where load fluctuations are not so severe as to overly stress the spring or springs. A conventional blade spring tensioner includes a blade shoe having a curved chain sliding face which maintains contact with the strand of chain with which it is engaged. To increase the amount of tensioning force applied to the chain, at least one blade spring is installed between the blade shoe and the chain sliding face. A bracket houses the blade shoe and the chain sliding face. The bracket is securely mounted to the engine by bolts, rivets or other such means. There may be only one mounting means which would allow the bracket to pivot in response to changing tension loads. The pivot point may be at either end of the bracket or in the middle, as desired. Alternatively, the bracket may be securely mounted to the engine by two or more mounting means which effectively prevents any pivoting movement of the tensioner. In any case, the mounting means are located adjacent the strand of chain with which the tensioner is engaged. Often, the mounting means are located outside of chain loop itself.
FIG. 1 shows a prior art chain driving system having a blade tensioner and a guide. A closed loop chain 8 encircles driving sprocket 12 and driven sprocket 10. Each sprocket 10 and 12 accelerates and decelerates independently while maintaining forward motion. A fixed guide 14 is attached to a bracket 7 on the tight strand of the chain. Opposite the guide 14 on the slack strand of the chain is a tensioner 16, which is at least semi-rigidly fixed to the bracket 7 and biased towards the tight strand of the chain. Bolts 18 fasten the bracket containing the tensioner 16 and the guide 14 to the engine block (not shown).
When the driving sprocket 12 accelerates or the driven sprocket 10 decelerates, an energy wave or high local load is created in the tight strand of the chain, which travels from the sprocket that has changed in velocity toward the other sprocket. The chain 8 attempts to span the distance between the link of the chain in contact with the initiating sprocket to the other sprocket in the shortest possible distance, a straight line. The energy moves through the links on the free strand of the chain until it comes to the end of the guide 14, which absorbs the energy. As a result of the constant absorption of the high local load the ends of the guide 14 sustain significant wear. The energy wave can also be present and provide the same results if it were to have originated in the slack strand. However, the effects of these energy waves may not occur at all depending on how well the tensioner keeps the slack out of the chain.
U.S. Pat. No. 5,967,922 discloses a tensioning device containing a tension lever that has first and second slide blocks affixed such that the slide blocks are on either side of the slack side of the chain. The first slide block is pivotally attached to a guide rail on the tight side of the chain. The second slide block is biased against one of the sides of the slack side of the chain by a tension element comprising a compression spring and a piston.
U.S. Pat. No. 6,322,470 discloses a tensioner that includes a pair of pivoting arms used to simultaneously tension two separate strands of the same chain. A lever with fixed pins is located between the two strands. The arms are pivotally mounted to the fixed pins and extend outside the strands of the chain. They contain shoes to contact the outside portion of the chain. Rotation of the lever causes the fixed pins to move laterally and draw in the arms to simultaneously impart tension to the separate strands of chain.
Japanese Application No. 2003-074652 discloses a tensioner in which a support member is pivotally located on a center line connecting the two sprockets. Attached to the support member are pressing members on either side of the chain. The damping that is imparted to the slack and tight strands of the chain is individual to each strand because each of the pressing members floats relative to the chain. While there is only one pivot present between the two pressing members, the load on one tries to affect the other and the pressing members undulate back and forth on the chain.
Referring to FIG. 2, a chain tensioner assembly as disclosed in US Patent Publication No. 2005/0085322A1 is shown. The chain tensioner assembly consists of a tensioner 116 and a chain guide 114, both of which are secured to a bracket 136. The tensioner 116 engages one strand of chain 108 while the chain guide 114 engages the other strand of the chain. The bracket is pivotally mounted to the engine housing at a pivot means 120 which is located in a hole 128 between the two strands of the chain and along the centerline that is formed between the central axis of the driving sprocket 112 and the central axis of the driven sprocket 110. The bracket is allowed to pivot about the pivot means 120 in either a clockwise or counterclockwise direction in response to both slack and tight tension conditions experienced by either strand of the chain.
A typical closed loop chain drive power transmission system will have one strand that is tight most of the time during operation while the other strand is slack most of the time. The prior art tensioners address this condition by placing a tensioning device in contact with the strand that is predominantly slack while merely placing a guide element adjacent the tight strand. These devices are often not capable of providing a smooth transition from the predominant tension condition to the opposite tension condition in which the tight strand becomes slack and the slack strand becomes tight. It is desirable to have a tensioner that can smoothly adjust to a reversal in the predominant tension condition of the chain so that a balanced tension is experienced on a consistent basis throughout the entire chain system.